<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      const n = +prompt('数量')
      function getSum(n) {
        if (n <= 1) {
          return 0
        } else if (n <= 3) {
          return 1
        }
        const sum = parseInt(n / 3)
        const left = n % 3
        return sum + getSum(left + sum)
      }
    console.log(getSum(n));
    </script>
  </body>
</html>
